1. Technical Field
The invention relates to an optical isolator for use with fiber-optic cables and, more particularly, to a polarization independent optical isolator having cables on one sides only and using a reflector to produce two stages of isolation.
2. Background
It is well known in the art that a polarization independent optical isolator is an essential device for optical fiber communication systems that use laser sources. An optical isolator is an optical component that transmits an optical signal in a forward direction and blocks (isolates) it from transmitting in a backward direction. In telecommunications systems, the input and output to the device are provided by fiber-optic cables that interface with other devices. Backward reflections from devices to which the output cable connects creates the need to isolate these reflections from devices to which the input cable connects.
There are two physical configurations. In an in-line configuration, the input and output cables are on opposite sides and light passes through the isolator from one end to the other. In a single-sided configuration, the input and output cables are on the same side. Generally, a reflective element is opposite the cables directing light from one to the other through the isolator. Single-sided configurations are preferred in some installations where there are a large number of isolators and other devices. It is easier to locate the fiber-optic cables if they enter and leave from the same side. In still other applications, the reflective element can be designed to allow a small amount of the light going though isolator to be tapped off to monitor the light intensity.
Isolators should have a number of desirable characteristics. The transmission from the input cable to the output should be as high as possible, i.e., the insertion loss should be low. The transmission from the output cable to the input should be as low as possible, i.e., the isolation should be high. The light in the input cables may be randomly polarized and it is necessary that the isolator produces the same low insertion and high isolation independent of polarization. Also, whatever the polarization in the input cable, an isolator usually resolves the light into two orthogonal polarizations in the first element. In order to prevent short pulses from being broadened, it is desirable to maintain the same phase relationship between these two polarizations in the output cable, i.e., there should be no dispersion between the phases of the two polarizations.
Commercial devices are preferably compact, inexpensive, and easy to put together and align. For instance, a number of isolators use wedged shaped optical elements. A design using flat plates should be more convenient to manufacture and assemble. One of the problems in making isolators is that a high degree of isolation requires precision alignment. Even with precision alignment, the optical properties of some of the components arc a function of wavelength and temperature so that isolator performance degrades for wavelengths and temperatures away from the ones used for the design.
In some applications an isolation of 60 dB (one part per million) is desired. This is almost impossible to achieve with a single isolator and the usual solution is to cascade isolators in a series of stages. On stage is a complete isolator in itself and this feeds a second complete isolator. Using an in-line configuration, in principle, this is straightforward. For example, the output cable of an isolator with an isolation of 30 dB can be used as the input of cable of a second isolator to yield 60 dB of isolation. Usually, however, two stage in-line isolators do not use separate cables to connect the two stages, but put them together in one package. Making in-line isolators is eased somewhat because the light beams are aligned along one axis. Two stage in-line isolators with 55 dB of isolation are commercially available.
In spite of being preferable in some applications, single-sided isolators are less common. There does not seem to be any commercially available two-stage single-sided isolators, at least ones with greater than 35 dB of isolation. This may be because single-sided isolators are harder to make. One challenge with all two-stage isolators is to provide two stages of isolation for even spurious light beams due to imperfect optical components and alignment. Although the optical isolator field is very crowded with many designs using the same or similar components in a variety of configurations, a practical two-stage single-sided isolator is not yet available.